Tunnel heterojunctions in Group IV/Group II-IV multijunction solar cells

ABSTRACT

A photovoltaic cell comprises a first subcell formed of a Group IV semiconductor material, a second subcell formed of a Group II-VI semiconductor material, and a tunnel heterojunction interposed between the first and second subcells. A first side of the tunnel heterojunction is formed by a first layer that is adjacent to a top surface of the first subcell. The first layer is of a first conductivity type, is comprised of a highly doped Group IV semiconductor material. The other side of the tunnel heterojunction is formed by a second layer that adjoins the lower surface of the second subcell. The second layer is of a second conductivity type opposite the first conductivity type, and is comprised of a highly doped Group II-VI semiconductor material. The tunnel heterojunction permits photoelectric series current to flow through the subcells.

BACKGROUND OF THE INVENTION

To achieve high energy conversion efficiency for a semiconductorphotovoltaic solar cell, a high output voltage and a high current arerequired. In order to take advantage of narrow and wide band gapphotovoltaic materials, a multijunction photovoltaic solar cellarchitecture approach has been proposed in which the cell includes anumber of stacked photovoltaic solar subcells each with different baselayer energy gaps. By connecting the photovoltaic solar cells in aserial fashion with the base layer energy gaps covering differentportions of the solar spectrum, enhanced energy conversion efficiencycan be achieved.

However, it has proven difficult to provide a path for a photogeneratedcurrent to pass from a Group II-VI semiconductor layer to a Group IVsemiconductor layer. For example, FIGS. 1A and 1B show that a blockingcontact is formed when a p-type ZnTe layer 100 is joined to either lightor moderately doped p-type silicon 102 or n-type silicon 104. As isillustrated in FIG. 2, since it is difficult to form stable n-type ZnTelayers, even a highly doped (n++) silicon layer 200 will not form anonblocking path from a Group II-VI layer 202 (such as ZnTe) to alightly to moderately doped p-type Group IV layer 204 (such as silicon).Thus, there is a need for a nonblocking path between adjacent subcellswhere these are formed of a Group IV semiconductor on the one hand and aGroup II-VI semiconductor on the other.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a photovoltaic cell comprisesa first subcell formed of a Group IV semiconductor material, a secondsubcell formed of a Group II-VI semiconductor material, and a tunnelheterojunction interposed between the first and second subcells. Thetunnel heterojunction includes a first Group IV semiconductor layer thatis adjacent to an upper surface of the first subcell. The first layer ishighly doped to assume a first conductivity type and forms one side ofthe tunnel heterojunction.

A second layer of the tunnel heterojunction adjoins the first layer andis adjacent to a lower surface of the second subcell. The second layeris highly doped to be of a second conductivity type opposite the firstconductivity type and forms the other side of the tunnel heterojunction.The second tunnel heterojunction layer is comprised of a Group II-VIsemiconductor material.

According to a second aspect of the invention, a method of passingphotovoltaic current from a second subcell formed from a Group II-VImaterial to a first subcell formed from a Group IV material comprisesthe step of forming a tunnel heterojunction between the first subcelland the second subcell. The step of forming the tunnel heterojunctionincludes the substep of forming a first layer in a Group IVsemiconductor material to be of a first conductivity type, and to beadjacent to a top surface of the first subcell. The step of forming theheterojunction further includes the substep of forming a second layer ina Group II-VI semiconductor material to be of a second conductivity typeopposite the first conductivity type. The second heterojunction layer isformed to be adjacent to a bottom surface of the second subcell.Hole-electron pairs generated by light incident on the cell may tunnelthrough the heterojunction between the subcells, thereby permitting theefficient generation of a photoelectric series current through bothsubcells.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the invention and their advantages can be discernedin the following detailed description, in which like characters denotelike parts and in which:

FIG. 1A is a highly magnified schematic elevational sectional viewshowing the interface of a p-type Group II-VI semiconductor layer with ap-type Group IV semiconductor layer and showing that their interface isa blocking contact;

FIG. 1B is a highly magnified schematic elevational sectional viewshowing the interface of a p-type Group II-VI semiconductor layer withan n-type Group IV semiconductor layer and showing that their interfaceis a blocking contact;

FIG. 2 is a highly magnified schematic elevational sectional view of ap-type Group II-VI semiconductor layer, a p-type Group IV semiconductorlayer, and an interface positioned therebetween of a degenerately dopedn-type Group IV semiconductor layer, showing that a photogeneratedcurrent cannot cross the interface;

FIG. 3 is a highly magnified schematic elevational sectional view of aportion of a multijunction photovoltaic cell according to a firstembodiment of the present invention;

FIG. 4 is a highly magnified schematic elevational sectional view of aportion of a multijunction photovoltaic cell according to a secondembodiment of the present invention;

FIG. 5 is a highly magnified schematic elevational sectional view of aportion of a multijunction photovoltaic cell according to a thirdembodiment of the invention; and

FIG. 6 is a flow diagram showing a method for forming photovoltaic cellto include a tunnel heterojunction between a Group II-VI subcell and aGroup IV subcell.

DETAILED DESCRIPTION

The invention relates to photovoltaic cells and methods of passingcurrent from a subcell having a Group II-VI semiconductor material to asubcell formed of a Group IV semiconductor material through the use of atunnel heterojunction. Contemplated Group II-VI materials used hereininclude CdS, CdTe, CdSe, ZnTe, ZnSe, ZnS, MgTe, CdSeTe, CdZnTe, CdMnTe,CdMgTe, CdHgTe, and composites thereof. Contemplated Group IV materialsused herein include silicon, germanium, strained silicon germanium, andsilicon-germanium. Preferred materials systems include ZnTe for theGroup II-VI structures described herein and elemental silicon for theGroup IV structures described herein.

Referring to FIG. 3, a multijunction photovoltaic cell, indicatedgenerally at 300, comprises a first subcell 302 formed of a Group IVsemiconductor material, a second subcell 304 formed of a Group II-VIsemiconductor material that is optically coupled to the Group IV subcell302, and a tunnel junction 306 interposed between the first and secondsubcells 302, 304. The photovoltaic cell 300 is so constructed thatincident light first enters the second, Group II-VI subcell 304, where arelatively energetic portion (such as a bluer portion of the solarspectrum) gets converted into hole-electron pairs. The remainder of theincident light continues through the tunnel junction 306 and into thefirst, Group IV subcell, where a redder portion of the incident lightwill also generate hole-electron pairs. It is an objective of theinvention to provide a nonblocking path between the subcells 304 and302, inducing a photoelectric series current Ito flow through cells 304and 302.

The tunnel junction 306 includes a first layer 308 that is adjacent toan upper surface 310 of the first, Group IV subcell 302. The first layer308 comprises Group IV semiconductor material and is highly doped suchthat layer 308 forms one side of the tunnel heterojunction. Layer 308has a first conductivity type, in this illustrated embodiment chosen as(n). A second layer 312 adjoins the first tunnel junction layer 308 andis adjacent to a lower surface 314 of the second subcell 304. The secondlayer 312 comprises a Group II-VI semiconductor material that has beenhighly doped so that layer 312 forms the other side of the tunnelheterojunction. Layer 312 has a second conductivity type (here, (p))that is opposite the first conductivity type. To permit the quantumtunneling of carriers between the sides of the tunnel heterojunction306, it is necessary that tunnel junction 306 be thin, such as 0.005 to0.1 μm. A thin tunnel junction will also reduce optical power loss. Thefirst subcell 302 has a p-type base 316 and an n-type emitter 318. Thesecond, Group II-VI subcell 304 has a p-type base 320 and an n-typeemitter 322. In another, nonillustrated embodiment, the depictedconductivity types can be reversed, such that the bases are n-type, theemitters are p-type, and the tunnel heterojunction has a p++ lower layerand an n++ upper layer. This reversal of conductivity types from theillustrated embodiment may require the use of II-VI semiconductor alloysthat are different from those (such as ZnTe) preferred to be used in theillustrated embodiment.

The cell 300 can be finished as a tandem solar cell with only twosubcells 302, 304. In this instance, a back contact (not shown) may beformed to adjoin a bottom surface of the Group IV base 316. Anantireflection layer and a front contact (neither shown) may be formedto be adjacent an upper surface of the Group II-VI emitter 322.

In an alternative embodiment shown in FIG. 4, a multi junctionphotovoltaic cell indicated generally at 400 has a first subcell 402, atunnel heterojunction 306, and a second subcell 304. The structure ofthe second, Group II-VI subcell 304 and of the tunnel heterojunction 306is the same as that described in the embodiment illustrated in FIG. 3.The first, Group IV subcell 402 is formed by a base 404 that is lightlydoped to be of the second conductivity type (here, (p)) and by theadjacent heterojunction layer 308. The (n) doping of heterojunctionlayer 308 should be sufficient for a pronounced change in the energylevels of the valence and conduction bands to occur between the (p) and(n) sides of the heterojunction, thereby permitting tunneling. Thedoping level may be moderate to high. In this embodiment, layer 308 actsas the emitter of cell 402 as well as one side of heterojunction 306.

Both cell 300 in FIG. 3 and cell 400 in FIG. 4 may additionally includefront and back contacts and an upper antireflection layer (described inmore detail herein in conjunction with the embodiment shown in FIG. 5).

FIG. 5 illustrates an embodiment of the invention which includes morethan two subcells. The illustrated cell 448 has a bottom subcell 302formed of a Group IV semiconductor material such as silicon, a tunnelheterojunction 306 that is formed of a layer each of Group IV and GroupII-VI semiconductor material, a middle subcell 304 that is formed ofGroup II-VI semiconductor material, a tunnel homojunction 454 formed ofGroup II-VI semiconductor material, and a top subcell 460 that is formedof a Group II-VI semiconductor material.

The bottom subcell 302 includes a (p) base 316 and an (n) emitter 318.The tunnel heterojunction 306 includes an (n++) layer 308 consisting ofa highly doped Group IV semiconductor material, and on top thereof a(p++) layer 312 consisting of a highly doped Group II-VI semiconductormaterial. The second subcell includes a (p) base 320 and an (n) emitter322. Formed on top of the second subcell is an (n++) layer 450 and a(p++) layer 452, both formed of a Group II-VI semiconductor material.The third subcell 460 includes a (p) base 456 and an (n) emitter 458.The Group II-VI semiconductor material making up layers 456 and 458preferably is chosen to have a higher band gap than the Group II-VIsemiconductor material making up layers 320 and 322. This illustratedembodiment also shows how an antireflective coating 464 and a conductivetransparent top contact 462 may be formed above the third subcell 460,and how a conductive bottom contact 466 may be formed to adjoin a bottomsurface of the (p) layer 316. Antireflective coatings and top and bottomcontacts may be added to the other embodiments illustrated herein. Inone variation of the embodiment shown in FIG. 5, all of the conductivitytypes may be reversed, with the understanding that this may require theuse of different Group II-VI alloys. In another variation of theembodiment shown in FIG. 5, the Group IV (n) emitter 318 may be omitted(n++ region 308 then acting as the emitter). In other embodiments,further subcells, preferably of Group II-VI semiconductor materials,could be formed above subcell 460, and preferably each of these subcellswill be separated from neighboring subcells by tunnel junctions.

Referring to FIG. 6, steps in a representative fabrication method areshown. In a preferred fabrication sequence, the lower, Group IV subcellis formed first. Semiconductors formed with silicon typically are morerobust and can better endure steps in semiconductor fabricationsequences than can the typically more delicate Group II-VI compoundsemiconductors, which therefore are preferred to be fabricatedsubsequently.

In the illustrated method, at step 500 a (p) type substrate or wafer ofGroup IV semiconductor material, such as Si, SiGe or strained SiGe, isprovided. Then, in one alternative, at step 502 a moderately to highlydoped (n) emitter is created in an upper portion of the Group IVsubcell, as by implantation or epitaxial growth on the (p) substrate. Inanother alternative this step is skipped. If created, the emitter layercould be doped to n or n+, but preferably may be doped to aconcentration that is less than that used for the n++ heterojunctionregion.

In either alternative the next step is to create a highly doped (n+ ton++) Group IV lower layer of the tunnel heterojunction, at step 504.This layer may be formed by diffusion, ion implantation or growth of afurther epitaxial layer. In the case of diffusion, this can be createdwith either As or P as a dopant at a temperature in the range of 850 to1000 C using rapid thermal annealing (rta), or a short furnace annealfrom a few to about 10 seconds. Alternatively, high dose implantations,with appropriate implantation energies, of either phosphorus or arsenicwith a short rta, or with a short furnace anneal in the range of a fewto about ten seconds, and at a temperature in the range of about 850 and950 C, could create this (n++) layer. In another alternative the (n++)layer could be grown epitaxially.

At step 505 (which may occur at any point after step 500 but before anystep in which Group II-VI layer(s) are formed) a bottom contact isformed from the bottom of the p-type Group IV semiconductor to a highconductance metallic layer. The bottom surface has a dopant, such asboron or other shallow acceptor, diffused or ion-implanted into it tocreate a (p++) layer. Then a metallic layer such as aluminum isdeposited on the surface.

At step 506, a Group II-VI upper layer of the heterojunction is created.This layer, which preferably is (p++) ZnTe, can be formed by introducingN during growth of the ZnTe layer, although other dopants that arep-type with respect to ZnTe (such as As or P) could be used.

At step 508, the second, Group II-VI subcell is formed on top of theupper heterojunction layer. This and further subcells (and necessaryintervening tunnel junctions) can be fabricated by material growthmethods such as MBE, MOCVD, and LPE. Layer thicknesses and alloying ofthe semiconductor thin layers that form the subcells of themultijunction photovoltaic solar cell can be controlled and optimized byadjusting the different raw material compositions, flux rates anddeposition durations during the growth to meet the requirements of thespecific design for a photovoltaic solar cell.

The resultant multijunction photovoltaic cell is one in whichphotogenerated current can be passed from a Group II-VI subcell, througha nonblocking path provided by the tunnel heterojunction of theinvention, to a Group IV subcell.

In summary, several multijunction photovoltaic cells have been proposedin which at least one of the subcells is formed of a Group IVsemiconductor material, at least one of the subcells is formed of aGroup II-VI semiconductor material, and a heterojunction is formedbetween them to provide a nonblocking contact. While illustratedembodiments of the present invention have been described and illustratedin the appended drawings, the present invention is not limited theretobut only by the scope and spirit of the appended claims.

We claim:
 1. A photovoltaic cell, comprising: a first subcell formed byan epitaxial growth process of a single crystal Group IV semiconductormaterial and having a first upper surface; a tunnel heterojunctionincluding a first layer comprising a single crystal Group IVsemiconductor material, the first layer having a first conductivity typeand formed by an epitaxial growth process on the first upper surface ofthe first subcell, the first layer forming one side of the tunnelheterojunction, the first layer having a second upper surface; and asecond layer comprising a single crystal Group II-VI semiconductormaterial, the second layer having a second conductivity type oppositethe first conductivity type, the second layer formed by an epitaxialgrowth process on the second upper surface of the first layer, thesecond layer forming the other side of the tunnel heterojunction, andhaving a third upper surface; and a second subcell formed by anepitaxial growth process on the third upper surface of the second layer,the second subcell formed of a single crystal Group II-VI semiconductormaterial.
 2. The cell of claim 1, wherein the first conductivity type is(n).
 3. The cell of claim 1, wherein the first conductivity type is (p).4. The cell of claim 1, wherein the first subcell comprises an emitterhaving the first conductivity type and a base having the secondconductivity type and being adjacent to the emitter.
 5. The cell ofclaim 1, wherein the first subcell comprises a base formed to be of thesecond conductivity type and adjoining the first layer of the tunnelheterojunction, the first layer of the tunnel heterojunction acting asan emitter of the first subcell.
 6. The cell of claim 1, wherein thesecond subcell is formed from a single crystal Group II-VI semiconductormaterial that is selected from the group consisting of CdS, CdTe, CdSe,ZnTe, ZnSe, ZnS, MgTe, CdSeTe, CdZnTe, CdMnTe, CdMgTe, CdHgTe, andcomposites thereof.
 7. The cell of claim 1, wherein the first subcell isformed from a single crystal Group IV semiconductor material that isselected from the group consisting of silicon, strainedsilicon-germanium, and silicon-germanium.
 8. The cell of claim 1,wherein the first layer of the tunnel heterojunction comprises a singlecrystal Group IV semiconductor material selected from the groupconsisting of silicon, strained silicon-germanium, andsilicon-germanium.
 9. The cell of claim 1, wherein the second layer ofthe tunnel heterojunction comprises a single crystal Group II-VIsemiconductor material selected from the group consisting of ZnTe, ZnS,MgTe, CdMgTe and composites thereof.
 10. The cell of claim 1, whereinthe second layer is more highly doped than an adjacent portion of thesecond subcell.
 11. The cell of claim 1, wherein the first layer is morehighly doped than an adjacent portion of the first subcell.